My wife wore her Polar heart monitor on a recent 10-mile day-hike and found that she burned 5600 calories during the day. If I follow the rule of thumb at 1.4ppppd, she will be consuming around 2800 to 3000 calories/day, which doesn't come anywhere near the 5600 the monitor said she burned.

My approach is to gain 3-4# of body fat in the 3-4 weeks before a long trip and use it to make up the difference. It has worked for me up to 11 days so far, but at some point body fat will be used up and you will begin to cannibalize muscle tissue. In my case, I hypothesize that would happen at around 14 days based on my experience so far. There have been several fairly long threads on exactly this subject so far, mostly in "Food, Hydration, and Nutrition", but also, IIRC, in Philosophy and Technique. They would be well worth locating and reading. Also, RJ did a pretty good writeup on the subject on his Arctic1000 website prior to the Arctic1000 expedition. I am sure your query will get a lot of replies before long, so stay tuned. Lots of folks are wrestling with this problem and there are a lot of good ideas floating around in the community.

Your other option, BTW, is to carry more food, but that will limit the duration of your trips in short order if you try to carry 5600 calories of food/day.

5600 calories is a generic Calculation by Polar. The amount of calories your wife burned is best found through experience. [Edit: 5 Hours X 400 calories = 2000 calories]

As mentioned by Tom, body fat is the principle reserve we work against, and most of us have enough to supplement on multi-day hikes. Rations for military field days are around 4500 calories. Ripped from Livestrong: "A 130-pound person will burn approximately 590 calories in one hour of heavy backpacking, the Changing Shape website reports. The same person will only burn approximately 295 calories in one hour of light backpacking." And,if I recall, Mike C! said NOLS trips (multi-week) aim at 3300 calories per day.

Another source, when you are looking at a one-day event, is the amount of reserves you have stored in your muscles and in your liver. This is generally around 1500 calories for a "typical guy" with lots of muscle mass. I do not recall what the number is for a "typical woman" (Probably around 1200 calories.)

For a 7 day trip you also have the benefit of a "caloric cushion" that usually lasts for 3 to 4 days for me. After that I Know when I'm hungry. But if I'm hiking 10 hours a day hunger is typically not at the front of my mind, and I do just fine on "fewer calories in versus calories out". (It is on the "social trips" with lots of camp time that I suffer.)

So take 3000 calories a day. Weigh in and weigh out. See how you do. See how much food you bring back. Keep good records and adjust as needed. And, as always, YHOH.

That would not be difficult to estimate if all foods were either pure fat, pure protein, or pure carbohydrate. However, all of our foods are mixtures of varying degree. I've seen charts of how many calories typical foods hold, but I am always dealing with some weird concoction that fails to be charted. This forces me to make wild guesses about calorie content, or else skip the whole process. I'll carry about 1.5 pounds of food per day of something that vaguely resembles a healthy mix, and then I stop right there.

On my recent trip, I was out for three nights (less than 72 hours). I carried out 5.5 pounds of food, and I returned with 2 pounds of food (because I didn't know how long I was going to be out). I lost 7 pounds of body weight, but I think half of that loss was from dehydration.

"That would not be difficult to estimate if all foods were either pure fat, pure protein, or pure carbohydrate. However, all of our foods are mixtures of varying degree."

Packaged foods all have nutrition numbers, usually related to ounces and grams.

For food I put together at home I use CalorieKing to determine my mix of carb, protein, and fat, as well as total calorie count. It lets you search by food name, commercial name, and brand name. A accurate kitchen scale is essential to the process.

Probably more important the C/P/F ratio is just taking food you like to eat. Unless you are trying to fastpack 25 to 30 miles a day, just about anything you can get down will work as long as the calories add up.

Initially I used a spreadsheet to see how my food was balanced and what the total calorie count came to. Now I know from experience where to adjust.